Analog bias control of rf amplifiers

ABSTRACT

Examples provide methods and apparatus for controlling a DC bias current in an RF amplifier. In one example where the RF amplifier is implemented on an amplifier die, a reference voltage is produced across a reference resistor implemented on the amplifier die, the DC bias current is measured, and a current controller, which is implemented on a controller die that is separate from the amplifier die, operates a feedback loop using the reference voltage to control a level of the DC bias current.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Patent Application Ser. No. 62/905,590, titled ANALOG BIASCONTROL OF RF AMPLIFIERS, filed Sep. 25, 2019, the content of which isincorporated herein by reference for all purposes.

BACKGROUND

The DC bias current in certain amplifiers, such as those used in CATVapplications, needs to be set and controlled tightly in order to meetperformance specifications. The conventional approach to controlling theDC bias current relies on laser trimming DC bias resistors on thecircuit board or die during production (manufacture) of the amplifiermodules, for example, during the die sorting phase. This process isexpensive, and as applications start to require increased total current(for example, on the order of about 760 milliamps (mA)) and DC powerdissipation, may not be accurate enough to meet stringent performancespecifications. For example, in an application where the bias current isaround 760 mA, a variation of even 10% (˜76 mA) may be too much to meetthe relevant specifications.

SUMMARY OF INVENTION

Aspects and embodiments are directed to control circuitry that canprovide adjustable control of the DC bias current in amplifiers andeliminates the need for costly on-die laser trimming to set the current,whether at the die sorting phase or another stage during manufacture.

According to one embodiment, an amplifier module comprises a radiofrequency (RF) amplifier, a current sensing resistor connected to the RFamplifier, a sense voltage measured across the current sensing resistorbeing indicative of a DC bias current in the RF amplifier, a referenceresistor, and a current controller including a current source and afeedback amplifier, the feedback amplifier having an output connected toa gate node of the RF amplifier, a first input connected to thereference resistor, and a second input connected to the current sensingresistor, the current source being configured to apply a first currentto the reference resistor to produce a reference voltage at the firstinput of the feedback amplifier, the feedback amplifier being configuredto produce a control voltage at the gate node to drive the RF amplifierto adjust the DC bias current to equalize the sense voltage and thereference voltage.

In one example, the output of the feedback amplifier is connected to thegate node of the RF amplifier via a gate resistor.

In another example, the RF amplifier, the current sensing resistor, andthe reference resistor are implemented on an amplifier die, and whereinthe current controller is implemented on a controller die separate fromthe amplifier die.

In one example, the current controller includes a first filter connectedto the output of the feedback amplifier. In one example, the firstfilter includes a first series resistor connected between the output ofthe feedback amplifier and the gate node, and a first shunt capacitorconnected between the first series resistor and ground. In anotherexample, the first filter includes a second order resistive-capacitive(RC) filter.

In one example, the RF amplifier is a differential amplifier.

In another example, the current controller includes chopper circuitrycoupled to the feedback amplifier and configured to reduce an offsetvoltage of the feedback amplifier.

In another example, the current controller has at least oneuser-adjustable parameter, and wherein the current controller includes acontrol input configured to receive a control signal to adjust the atleast one user-adjustable parameter.

In one example, the current source is programmable and wherein a valueof the first current is controllable via a programming interfaceconnected to the current source.

In another example, the current controller includes a control inputconfigured to receive a control signal, and wherein a value of the firstcurrent is trimmable via the control signal.

According to another embodiment, an amplifier module comprises a radiofrequency (RF) amplifier implemented on an amplifier die, a senseresistor implemented on the amplifier die and connected to the RFamplifier, a sense voltage measured across the current sensing resistorbeing indicative of a DC bias current in the RF amplifier, a referenceresistor implemented on the amplifier die, and a current controllerimplemented on a controller die separate from the amplifier die, thecurrent controller being coupled to the RF amplifier, the sense resistorand the reference resistor, and configured to produce a referencevoltage across the reference resistor and to drive a gate voltageapplied to a gate node of the RF amplifier to control a DC bias currentin the RF amplifier based on the sense voltage and the referencevoltage.

In one example, the controller die includes a first contact forconnection to the reference resistor, a second contact for connection tothe sense resistor, and a third contact for connection to the gate nodeof the RF amplifier.

In one example, the current controller includes a feedback amplifierhaving an output connected to the third contact, a first input connectedto the first contact, and a second input connected to the third contact,the feedback amplifier being configured to receive the reference voltageat the first input, to receive the sense voltage at the second input,and to produce a control voltage at the output. In another example, theamplifier die includes a first amplifier contact for connection to theoutput of the feedback amplifier, and wherein the amplifier modulefurther comprises a gate resistor implemented on the amplifier die andconnected between the gate node of the RF amplifier and the firstamplifier contact. The amplifier module may further comprise a shuntresistor connected to ground in parallel with the reference resistor.The amplifier module may further comprise a pull-up resistor connectedfrom the reference resistor to a supply voltage.

In one example, the current controller further includes a first voltagereference and a voltage regulator, each connected to a supply contact onthe controller die and configured to receive a supply voltage via thesupply contact, the voltage regulator being further connected to asupply terminal of the feedback amplifier. In another example, thecurrent controller further includes programmable voltage referenceconfigured to receive outputs from the first voltage reference and thevoltage regulator, and output of the programmable voltage referencebeing connected to the first contact and configured to supply a drivecurrent to the reference resistor to produce the reference voltage. Thecurrent controller may further include a digital regulator, controllogic, and programming circuitry. In one example, the current controllerhas at least one user-adjustable parameter, and wherein the programmingcircuitry is configured to receive a control signal to adjust the atleast one user-adjustable parameter. The at least one user-adjustableparameter may include a temperature-dependence profile of the DC biascurrent, for example.

In one example, the programming circuitry is coupled to the programmablevoltage reference, wherein the programmable voltage reference includes aplurality of different operating modes, and wherein the programmingcircuitry is configured to select one of the plurality of operatingmodes based on the control signal. The at least one user-adjustableparameter may include a value of a reference current that produces thereference voltage across the reference resistor, for example.

In one example, the current controller further includes a filter coupledbetween the voltage regulator and the supply terminal of the feedbackamplifier. In one example, the filter includes a resistor connected inseries between the voltage regulator and the supply terminal of thefeedback amplifier and a shunt capacitor connected between the supplyterminal and ground.

In another example, the current controller further includes choppercircuitry coupled to the feedback amplifier and configured to reduce anoffset voltage of the feedback amplifier. In one example, the currentcontroller is further configured to compensate for the offset voltage byadjusting a value of a reference current that produces the referencevoltage across the reference resistor.

In another example, the RF amplifier is a differential amplifierincluding a pair of complementary transistor amplifiers, and a pair ofgate resistors connected between gates of the pair of complementarytransistor amplifiers and the gate node. In one example, the senseresistor is connected to a common virtual ground node between the pairof complementary transistor amplifiers. The amplifier module may furthercomprise a first resistive-capacitive (RC) filter implemented on thecontroller die and coupled between the output of the feedback amplifierand the third contact. In one example, the first RC filter includes afirst series resistor connected between the output of the feedbackamplifier and the third contact, and a first shunt capacitor connectedbetween the third contact and ground. In another example, the amplifierdie includes a first amplifier contact for connection to the output ofthe feedback amplifier, the amplifier module further comprising anexternal capacitor not implemented on either the amplifier die or thecontroller die, the external capacitor being coupled to the thirdcontact on the controller die and to the first amplifier contact on theamplifier die.

The amplifier module may further comprise a package configured to housethe controller die and the amplifier die, the package including aplurality of connection leads. In one example, the third contact on thecontroller die is connected to a first connection lead of the pluralityof connection leads of the package, the first amplifier contact isconnected to a second connection lead of the plurality of connectionleads of the package, and the external capacitor is connected to thefirst and second connection leads. In one example, a connection of theexternal capacitor to the first and second connection leads includes aKelvin connection. In another example, the first and second connectionleads are adjacent to one another.

In one example, first RC filter includes a second order RC filter and anadditional resistor connected in series between the second order RCfilter and the third contact.

The amplifier module may further comprise a second RC filter implementedon the amplifier die and coupled between the gate node and the firstamplifier contact.

Another embodiment is directed to a method of controlling a DC biascurrent in an RF amplifier implemented on an amplifier die. According toone embodiment, the method comprises producing a reference voltageacross a reference resistor implemented on the amplifier die, measuringthe DC bias current, and operating a feedback loop in a currentcontroller using the reference voltage to control a level of the DC biascurrent, the current controller being implemented on a controller dieseparate from the amplifier die.

In one example, measuring the DC bias current includes receiving a sensevoltage at the current controller, the sense voltage being producedacross a sense resistor by the DC bias current, the sense resistor beingimplemented on the amplifier die.

In another example, operating the feedback loop includes receiving thereference voltage at a first input of a feedback amplifier, receivingthe sense voltage at a second input of the feedback amplifier, producinga control voltage at an output of the feedback amplifier, and driving agate node of the RF amplifier with the control voltage to control thelevel of the DC bias current.

The method may further comprise filtering the control voltage.

In one example, producing the reference voltage includes generating areference current with the current controller, and applying thereference current to the reference resistor.

The method may further comprise trimming a value of the referencecurrent to compensate for an offset value of the feedback amplifier.

In one example, the method further comprises adjusting atemperature-dependence profile of the DC bias current using the currentcontroller.

In another example, the method further comprises adjusting the level ofthe DC bias current by altering a resistance value of the referenceresistor. In one example, adjusting the level of the DC bias currentincludes lowering the level of the DC bias current by connecting a shuntresistor to ground in parallel with the reference resistor. In anotherexample, adjusting the level of the DC bias current includes raising thelevel of the DC bias current by connecting a pull-up resistor from thereference resistor to a supply voltage.

The method may further comprise adjusting at least one characteristic ofthe current controller via a control interface on the controller die.

Still other aspects, embodiments, and advantages of these exemplaryaspects and embodiments are discussed in detail below. Embodimentsdisclosed herein may be combined with other embodiments in any mannerconsistent with at least one of the principles disclosed herein, andreferences to “an embodiment,” “some embodiments,” “an alternateembodiment,” “various embodiments,” “one embodiment” or the like are notnecessarily mutually exclusive and are intended to indicate that aparticular feature, structure, or characteristic described may beincluded in at least one embodiment. The appearances of such termsherein are not necessarily all referring to the same embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of at least one embodiment are discussed below withreference to the accompanying figures, which are not intended to bedrawn to scale. The figures are included to provide illustration and afurther understanding of the various aspects and embodiments, and areincorporated in and constitute a part of this specification, but are notintended as a definition of the limits of the invention. In the figures,each identical or nearly identical component that is illustrated invarious figures is represented by a like numeral. For purposes ofclarity, not every component may be labeled in every figure. In thefigures:

FIG. 1 is block diagram of one example of an amplifier and currentcontroller according to aspects of the present invention;

FIG. 2A is a block diagram of an example of a variation of the amplifierand current controller of FIG. 1, according to aspects of the presentinvention;

FIG. 2B is a block diagram of another example of a variation of theamplifier and current controller of FIG. 1, according to aspects of thepresent invention;

FIG. 3 is a block diagram of one example of a current controlleraccording to aspects of the present invention;

FIG. 4 is a schematic diagram of an example of a current controller anddifferential amplifier according to aspects of the present invention;

FIG. 5 is a block diagram of one example of a chop-stabilized feedbackamplifier for use in a current controller according to aspects of thepresent invention;

FIG. 6 is a schematic diagram of another example of a current controllerand differential amplifier according to aspects of the presentinvention;

FIG. 7 is a schematic diagram of another example of a current controllerand differential amplifier showing an example of filtering aspectsaccording to aspects of the present invention;

FIG. 8 is a diagram of one example of an amplifier module including anRF amplifier and current controller according to aspects of the presentinvention;

FIG. 9 is a graph showing simulated performance of various filteringconfigurations for examples of the amplifier and current controllercombination of FIG. 7; and

FIG. 10 is a graph showing simulated effects of mutual inductance and aKelvin connection on filtering performance.

DETAILED DESCRIPTION

Accurate control of the DC bias current in radio frequency (RF)amplifiers used in a variety of applications, including CATVapplications, can be an important design requirement to meet performancespecifications. Aspects and embodiments are directed to controlcircuitry that can provide accurate, adjustable DC bias current control,for example, to within about 1%, and eliminate the need for costly (andoften inaccurate) on-die laser trimming of resistors, as isconventionally required. According to certain embodiments, the controlcircuitry is configured to set and hold the RF amplifier DC current,also referred to herein as the bias current, independent of deviceparameters (e.g., whether the amplifier uses bipolar junction transistor(BJT) devices or field effect transistor (FET) devices) and independentof the DC power supply voltage. In addition, as discussed below, thecontrol circuitry can be configured to set and hold the RF amplifier DCbias current independent of process resistor sheet resistance variationon the amplifier die. As discussed further below, the DC bias currentmay be adjustable by a user, allowing the same control module to be usedwith a variety of different amplifiers and in different applications.Further, as discussed in more detail below, the control circuitry can beconfigured to keep the DC bias current constant even if the active RFdevice parameters change over time (device aging), and over a range ofdifferent RF output power levels. In addition, in certain embodiments,the RF amplifier bias current variation with temperature isprogrammable, which may be useful to maintain a desired distortionprofile over temperature. The current control circuitry and techniquesaccording to embodiments disclosed herein provide a flexible, robustapproach for setting and maintaining a desired DC bias current that canapplied to a variety of different RF amplifier architectures and over awide range of operating conditions (e.g., a range of RF power levelsand/or desired DC current levels, changing temperature, etc.).

It is to be appreciated that embodiments of the methods and apparatusesdiscussed herein are not limited in application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the accompanying drawings. Themethods and apparatuses are capable of implementation in otherembodiments and of being practiced or of being carried out in variousways. Examples of specific implementations are provided herein forillustrative purposes only and are not intended to be limiting. Also,the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use herein of“including,” “comprising,” “having,” “containing,” “involving,” andvariations thereof is meant to encompass the items listed thereafter andequivalents thereof as well as additional items. References to “or” maybe construed as inclusive so that any terms described using “or” mayindicate any of a single, more than one, and all of the described terms.Referring to FIG. 1, there is illustrated a block diagram of one exampleof a current controller 100 in combination with an RF amplifier die 200.As shown, the current controller 100 includes an operational amplifier(“op-amp”) 110 and a current source 120. The current controller 100 isimplemented on a controller die 130, which may be a CMOS die, forexample. In certain examples, the controller die 130 is dedicated to thecurrent controller 100; in other examples, the controller die 130 may beshared by the current controller 100 and other circuitry and/orcomponents used for other functions in an overall module or device inwhich the current controller 100 is used. The controller die 130includes at least three contacts (e.g., pins or pads) 132, 134, 136, forconnecting the current controller 100 to the amplifier die 200, asdiscussed further below. The amplifier die 200 includes an RF amplifier,represented schematically in FIG. 1 by amplifier (transistor) 210. Thoseskilled in the art will appreciate that the RF amplifier 210 in practicemay include a plurality of transistors and/or multiple amplificationstages, and the single transistor shown in FIG. 1 is intended to berepresentative only and not limiting in terms of implementation of theRF amplifier 210. In certain examples, such as a cascode amplifierconfiguration, for example, the RF amplifier 210 may represent thecommon source transistor(s), or DC current-setting stage, of the overallamplifier architecture. The amplifier die 200 further includes a currentsensing resistor 220 (Rs) and a reference resistor 230 (Rref). Again,the current sensing resistor 220 and the reference resistor 230 areschematically represented in FIG. 1 by single resistors, but may each beimplemented in practice by any configuration or network of one or moreresistive components. The amplifier die 200 may be a GaN, GaAs, or pHEMTdie, for example.

The current source 120 on the controller die 130 forces a current intothe reference resistor 230 on the amplifier die 200. This creates avoltage (Vref) across the reference resistor 230 that is applied at oneof the inputs of the op-amp 110. The other input of the op-amp 110receives a sense voltage (Vs) across the current sensing resistor 220.The output of the op-amp 110 is connected to the gate of the RFamplifier 210 via a gate resistance 240, and increases or decreases involtage to make the voltage Vs equal to Vref. The DC bias current in theamplifier (Id) is equal to Vs/Rs. Thus, by controlling the voltage Vs,the control loop implemented by the op-amp 110 holds this current (Id)constant. Further, the value of the amplifier DC bias current, Id, canbe determined by measuring the voltage, Vs, provided that the value ofRs is known. In certain examples, the current source 120 may beprogrammable to a desired value, as discussed further below.

The value of the amplifier DC bias current, Id, can be set to a desiredamount by selecting a value of the reference resistor 230. In certainexamples, the bias current can be adjusted by a user. For example, auser can lower the amplifier DC current, Id, by connecting a shuntresistor 232 to ground in parallel with the reference resistor 230, asshown in FIG. 2A. Similarly, the user can increase the amplifier DCcurrent, Id, by adding a pull-up resistor 234 from the referenceresistor 230 to the supply voltage, Vdd, as shown in FIG. 2B. In certainexamples, the controller die 130 can be provided with a contact 138connected to the reference resistor 230 to which the user can connectthe shunt resistor 232 or pull-up resistor 234. The contact 138 may bethe same or different from the contact 132 shown in FIG. 1. In otherexamples, the amplifier die 200, instead of the controller die 130, canbe provided with the contact 138 to allow the user to connect the shuntresistor 232 or pull-up resistor 234 to the reference resistor 230 toadjust the value of Id. In addition, although not shown in FIG. 1 or2A-B, in certain examples, a shunt resistor may be connected to thecontact 134 in parallel with the sense resistor 220 to lower the DC biascurrent, Id. In all cases, once the desired value of the amplifier DCcurrent, Id, has been selected, for example, through selection of thevalue of the reference resistor 230 and any adjustment using the shuntresistor 232 or pull-up resistor 234, or adjustment of the senseresistor 220, the current controller 100 holds the current constant atthe set value.

The current controller 100 may set and hold the amplifier DC current,Id, independent of the parameters or configuration of the RF amplifier210. For example, the RF amplifier 210 may be implemented using BJT,FET, or pHEMT devices in a single-ended or differential configuration.The same current controller 100 can be used in any case and operates inthe same manner, regardless of the configuration of the RF amplifier210. The current controller 100 may also set and hold the amplifier DCcurrent, Id, independent of the DC power supply voltage (Vdd). Thecurrent controller 100 compensates for a variety of factors, such asprocess variations, drift or changing of the RF amplifier 210 parametersover time (device aging), and dependence of the RF amplifier 210threshold voltage on the DC supply or temperature, by sensing theamplifier bias current (Id) and controlling the gate voltage of the RFamplifier 210 to thereby maintain the amplifier bias current at thedesired value. As discussed above, the amplifier bias current, Id, issensed using the sensing resistor 220. As shown in FIG. 1, the sensingresistor 220 may be the RF amplifier degeneration resistor (connected onthe source/emitter), which provides broadband linear operation.

As the control loop implemented by the current controller 100 measuresthe actual amplifier DC current, Id, it also suppresses any bias changesresulting from changes in the RF output power level. Typically, inconventional RF amplifier designs, the amplifier DC current tends tovary at higher RF output power levels, which may be undesirable. Thecurrent controller 100 advantageously provides a solution to thisproblem. In addition, the current controller 100 may set and maintainthe amplifier DC current, Id, independent of process resistor sheetresistance variation on the amplifier die 200. Because both the sensingresistor 220 (used to measure the sense voltage Vs) and the referenceresistor 230 (used to measure the reference voltage Vref) areimplemented on the amplifier die 200, any variation in the sheetresistance, whether due to process variation, temperature changes, orother factors, affects the values of the sensing resistor 220 and thereference resistor 230 in the same way. For example, if the value of thesensing resistor 220 increases by 10% due to changes in the sheetresistance of the amplifier die 200 (rho), the value of the referenceresistor 230 increases by the same amount. Therefore, the amplifier DCbias current, Id, remains constant.

According to certain embodiments, the current controller 100 can beconfigured such that the RF amplifier DC bias current, Id, is fixed overtemperature, for at least a range of operating temperatures. Accordingto other embodiments, the current controller 100 can be configured suchthat the variation of the RF amplifier DC current, Id, is programmable.This may be beneficial to maintain desired distortion characteristics ofthe RF amplifier 210 over temperature. For example, the currentcontroller 100 can be configured such that the RF amplifier DC current,Id, can be programmed with a particular temperature coefficient tomaintain linearity of the RF amplifier 210 over temperature.

Referring to FIG. 3, there is illustrated a block diagram of one exampleof an implementation of the current controller 100 according to certainembodiments. The current controller 100 may include various componentsto implement the current source 120 and enable programming of certaincharacteristics or parameters of the current controller 100 and/or theRF amplifier DC current, Id. In the illustrated example, the currentcontroller 100 includes a voltage reference (bandgap) 115, a voltageregulator 125, and a programmable voltage reference 140 to implement thecurrent source 120. The voltage reference 115 and voltage regulator 125are both connected to a supply contact 131 through which the supplyvoltage (Vdd) is provided to the current controller 100. Theprogrammable voltage reference 140 receives the outputs from the voltagereference 115 and the voltage regulator 125. The voltage regulator 125is also connected to the supply contact of the op-amp 110 and regulatesthe supply voltage, Vdd, provided to the op-amp 110. The output of theprogrammable voltage reference 140 is connected to the contact 132 tosupply the current to the reference resistor 230 on the amplifier die200, as discussed above, and to one input of the op-amp 110 that is alsoconnected to the contact 132, as also shown in FIG. 1. As discussedabove with reference to FIG. 1, the other input of the op-amp 110 isconnected to the contact 134 to receive the sense voltage, Vs, and theoutput of the op-amp 110 is connected to the contact 136, which can beconnected to the gate of the RF amplifier 210 on the amplifier 200.

In certain examples, a capacitor 160 can be connected to the contact 131for filtering, as discussed further below. The controller die 130 mayfurther include one or more ground contacts 133 for connection to anexternal ground.

In the example of FIG. 3, the current controller 100 further includes adigital regulator 145, control logic 150, and programming circuitry 155.The digital regulator 145 regulates supply voltage/current provided fromthe voltage reference 115 to the control logic 150 and the programmingcircuitry 155. The control logic 150 may include a serial controlinterface for the current controller 100 to allow various parameters tobe tested and adjusted during manufacture of the current controller 100.For example, as discussed above, the current value provided by thecurrent source 120 may be programmable via the control interface. Inaddition, the current value of the current source 120 may be trimmableor adjustable through the control interface, whether or not it isprogrammable. In certain examples, programming may provide courseadjustment of the current value, while trimming provides fineadjustment. In other examples where the current value is fixed (i.e.,the current source 120 is not programmable), the current value may stillbe adjustable/trimmable through the control interface. The controlinterface may be a serial interface or a parallel interface. In someexamples, the control interface may be configured to use a communicationprotocol such as I²C, SPI, etc. In the illustrated example, thecontroller die 130 include two contacts 135 a, 135 b, corresponding to aserial clock line and a serial data line, respectively, connected to thecontrol logic 150 for receiving commands from externalcircuitry/components. These commands are used to set certain parametersof the current controller during manufacture, for example, at a testingor die sorting phase, so as to configure the current controller for aparticular application. Accordingly, in certain embodiments, thecontacts 135 a, 135 b may not be accessible after manufacture of thecurrent controller 100 is complete. The programming circuitry 155 can beused to program or “trim” various characteristics of the currentcontroller 100 during the manufacturing of the current controller 100.As discussed above, in certain embodiments, the temperature dependenceof the RF amplifier DC current, Id, (i.e., profile of changing currentvalue with changing temperature) is programmable. This can be achievedusing the programmable voltage reference 140 and the programmingcircuitry 155. For example, the programmable voltage reference 140 mayhave different operating modes, such as a mode in which the resulting RFamplifier DC current, Id, is constant with temperature and another modein which the value of the RF amplifier DC current, Id, has a slope overtemperature. The characteristics of the slope may be controlled bychanging the value of a temperature coefficient that describes theslope. Through the programming circuitry 155, the operating mode of theprogrammable voltage reference 140 may be selected and/or thetemperature coefficient may be selected or adjusted. Programmingcommands for the programming circuitry 155 may be received via thecontrol logic 150 and serial control interface discussed above. Theprogrammability of the programmable voltage reference 140 and theprogramming circuitry 155 may allow different embodiments of the currentcontroller 100 to provide different temperature profiles of the RFamplifier DC current, for example, to optimize performance for differentRF amplifier 210 configurations or different applications/modules inwhich the amplifier die 200 is used. As shown in FIG. 3, the currentcontroller 100 may include a contact 137 connected to the programmingcircuitry 155 to allow testing of the programming circuitry 155 (forexample, confirmation that the correct settings, operating mode of theprogrammable voltage reference 140, and the like have been programmed),during manufacturing. Accordingly, similar to the contacts 135 a, 135 b,the contact 137 may not be accessible after manufacture is complete.

According to certain embodiments, it is preferable for the op-amp 110 tohave a very low offset voltage, or that the offset voltage (Vos) istrimmed or “canceled out” during production of the current controller100. Accordingly, certain aspects are directed to trimming the offsetvoltage of the op-amp 110 to reduce the offset voltage to as close tozero as possible.

Referring to FIG. 4, there is illustrated a schematic diagram of oneexample of the current controller 100 in combination with an example ofa differential RF amplifier 300. In the example of FIG. 1, the RFamplifier is represented schematically by the transistor 210, asdiscussed above. In FIG. 4, the RF amplifier 300 is representedschematically by a pair of complimentary transistors 210 a, 210 b, withtheir gates connected together via gate resistors 242, 244. Each of thetransistors 210 a, 210 b is connected to a corresponding sensingresistor 220 a, 220 b, as shown. In both the examples of FIG. 1 and FIG.4, the RF amplifier DC bias current, Id, is given by the followingequation:

$\begin{matrix}{I_{d} = {{\frac{R_{ref}}{R_{s}}*I_{ref}} + \frac{V_{os}}{R_{s}}}} & (1)\end{matrix}$

As may be seen from Equation (1), the RF amplifier DC current, Id, isproportional to reference current (Tref) produced by the current source120, but is also affected by the offset voltage (Vos) of the op-amp 110.Accordingly, it is desirable to minimize Vos to ensure accurate settingof the desired current, Id. In certain embodiments, even small errors inthe RF amplifier DC current, Id, (e.g., ˜1% or 3.5 mA) can cause thegate voltage (VG) to drift to negative, adversely affecting theperformance of the RF amplifier 300. Therefore, in certain examples,even if a low-offset op-amp 110 can be selected for the design of thecurrent controller 100, the current controller 100 may be configured tofurther trim and reduce the offset to improve the accuracy of the RFamplifier DC current control. In certain examples where the trimming isperformed before the current controller 100 is connected to (packagedwith) the RF amplifier 300, the offset mitigation can be achieved bymeasuring the actual offset during die probe, for example, andcompensating for the offset when programming the current Id by alteringthe target value by Vos/Rs.

In certain embodiments, the programming circuitry 155 can be configuredto measure and trim the offset voltage of the op-amp 110 by adjustingparameters of one or more components of the current source 120, similarto as discussed above. In other examples, the current controller 100 canbe configured with a self-calibration loop to reduce the offset voltageand/or variations in the offset voltage. In certain examples, thesensing resistor 220 on the amplifier die 200 may have a relatively lowresistance value, and therefore any drift or other variation in theoffset voltage of the op-amp 110 can have a large impact on the biascurrent, Id. According to one embodiment, the current controller 100includes chopper circuitry to stabilize and remove, or reduce, theop-amp offset voltage, Vos.

FIG. 5 is a block diagram of one example of a chop-stabilized op-amp 110that can be used in the current controller 100. The chopper circuitryincludes switching circuits 112, 114 that are connected across theinputs (switching circuit 112) and outputs (switching circuit 114) ofthe op-amp 110, as shown. The switching circuits 112, 114 are configuredto rapidly switch (or reverse) the inputs and outputs of the op-amp 110back and forth. As a result, the off-set voltage, Vos, between theinputs rapidly changes from positive to negative and back, and thereforeaverages to approximately zero. The switching circuits 112, 114 operateat a rate or frequency, fc. The chopping technique may be continuous inthe time domain, and may reduce the op-amp offset voltage to less than 1μV, for example. In certain examples, use and/or operation of thechopper circuitry may be programmable. For example, through the controllogic 150 and programming circuitry 155, the offset voltage, Vos, may bemeasured, and the chopper circuitry can be enabled or disabled, and/orthe frequency, fc, may be altered to optimize performance based on, forexample, parameters of a given op-amp 110 and/or specifications for agiven application or module in which the current controller 100 is to beused. As shown in FIG. 5, in certain embodiments, a low pass filter(LPF) 410 may be included at the output of the op-amp 110 to reducenoise, such as ripples or harmonics at the op-amp output caused by thechopping.

As noted above, according to certain embodiments, the current controller100, and optionally also the amplifier die 200, can be configured withvarious filtering and other techniques to reduce noise and ensureaccurate, consistent setting and holding of the RF amplifier DC biascurrent, Id. For example, referring again to FIG. 4, in the case of adifferential RF amplifier 300, voltage sensing directly on the sensingresistors 220 a, 220 b may exhibit large RF voltage swings, and it isdesirable to minimize any RF signal in the sense voltage Vs.Accordingly, rather than measuring the sense voltage across either (orboth) sensing resistors 220 a, 220 b, a DC extraction circuit usingcommon mode resistors 222 and 224 is added on the amplifier die 200,connecting the two sensing resistor nodes together, as shown in FIG. 4.Thus, a “virtual ground” is created at the common node 226 wheredifferential RF voltage swings may be canceled out. The sense voltage,Vs, is measured across a common resistor 422 connected to the commonnode 226. In addition to using this common mode rejection to reduce theRF signal on the sense voltage, Vs, RC filtering can also be added. Forexample, referring to FIG. 6, an RC filter 420 can be added on theamplifier die 200 between the common node 226 at which the sensevoltage, Vs, is measured and the input of the op-amp 110. The RC filter420 includes a series combination of the common sensing resistor 422 anda capacitor 424. The input of the op-amp 110 can be connected betweenthe common resistor 422 and the capacitor 424 to receive the sensevoltage, Vs, and the capacitor 424 may shunt RF noise to ground.Although shown in FIG. 6 applied in the case of a differential RFamplifier 300, the RC filter may also be included in a single-ended RFamplifier configuration. For example, referring again to FIG. 1, acapacitor may be added in series with the sense resistor 220 (betweenthe sense resistor 220 and ground) to perform RF filtering. Furthermore,the capacitor 424 may alternatively be implemented on the controller die130, rather than the amplifier die 200. In other examples, the capacitor424 may be implemented using a combination of capacitive elements on theamplifier die 200 and the controller die 130.

In certain applications, it may be required for the RF amplifier tooperate over a very wide frequency range. For example, in CATVapplications, the RF amplifier may operate over a range of approximately50 MHz to 1.2 GHz. Accordingly, noise filtering on the any of thecurrent controller lines and between the current controller 100 and theRF amplifier 210/300 may be required to meet communications standardspecifications for noise, spurs, linearity, and the like. Examples ofnoise that may need to be filtered out include thermal noise of thecurrent controller 100, any switching noise from an internal or externalDC/DC regulated voltage supply, and any noise from the chopper circuitrydiscussed above. Referring again to FIGS. 4 and 5, as discussed above,in certain examples the current controller 100 may include a filter 410on the output of the op-amp 110 to reduce noise, such as ripples orharmonics at the op-amp output caused by the chopping circuitry. In theexample shown in FIG. 4, the filter 410 includes a resistor 412 and ashunt capacitor 414. However, in other examples (such as the exampleshown in FIG. 7 discussed below), the filter 410 may include multipleresistors 412 and/or multiple capacitors 414 in various configurations.

In addition to preventing noise from the current controller 100 frompassing to the RF amplifier 210/300 (e.g., being present on the gatevoltage, VG, of the RF amplifier 210/300), it may also be desirable toprevent RF noise, or leakage current (I_(LEAK)), from the RF amplifier210/300 from passing to the current controller 100. Accordingly,additional filtering circuitry, such as an RC filter, may be connectedbetween the current controller 100 and the RF amplifier 210/300. Incertain examples, filtering components may be included on the RFamplifier die 200. For example, referring to FIG. 4, in certainexamples, a resistor 432 may be connected in series with the resistor412 between a node 252 between the gate resistors 242, 244 (at which thegate voltage, VG, is supplied to the transistors 210 a, 210 b of the RFamplifier 300), and a shunt capacitor 434 may be connected between thenode 252 and ground, to provide an RC filter on the amplifier die 200.The embedded RC filter (e.g., resistor 432 and capacitor 434) on theamplifier die 200 may benefit from the low inductance of through-wafervias (TWVs) that may be typically used to connect various components onthe amplifier die 200 to the ground connection, and provide excellenthigh frequency filtering performance.

In certain examples, such as for GaAs- and GaN-implemented RFamplifiers, or other HBT/BJT, HEMT, or other transistor devices withgate current, RC filters may not be able to have very large R valuesbecause of the DC and RMS gate current from the RF amplifier 300 and theresulting voltage drop that may limit the bias range. As a result, incertain examples there is a need for smaller R values and large C valuesthat may be difficult or impracticable to integrate on the amplifier die200. In such cases, external discrete capacitors may be used. Forexample, referring to FIG. 7, a discrete external capacitor 440 may beconnected to the contact 136 on the controller die 130 and to acorresponding contact 202 on the amplifier die 200 in a shuntconfiguration between the output of the op-amp 110 and the RF amplifier300. The connections may be made using wirebonds 172, for example. Incertain examples, the external capacitor 440 may have a value in a rangeof 100 nF-1 μF.

According to certain embodiments, various techniques are used tomitigate parasitic inductance. For example, referring to FIG. 7, certainexamples employ the use of separate ground wire bonds 174 for thecurrent controller 100, along with an embedded RC filter 410 (asdiscussed above). In some examples, the wirebond(s) 172 connected to theexternal capacitor 440 add a low inductance value to the filter 410, andthe filter 410 uses one or more capacitors 414 with low C values (e.g.,50 pF), along with one or more resistors 412 as discussed above, toprovide high frequency filtering. In the example shown in FIG. 7, thefilter 410 includes a second order low pass filter for high frequencyfiltering, along with an additional resistor 416 to avoid anti-resonancewith the external capacitor 440. In addition, in certain examples, aKelvin connection to the external capacitor 440 may be used to minimizethe effect of the package and/or wirebond inductance. For example, theexternal capacitor 440 may have a large C value, as discussed above, toprovide low frequency filtering, and the Kelvin connection may be usedfor mid-range and high frequency filter optimization.

FIG. 8 is a diagram of one example of an amplifier module 500 with abuilt-in current controller 100. In this example, the RF amplifierincludes a pHEMT example of the differential amplifier 300 (on theamplifier die 200) as an output stage for a cascode amplifierimplemented on a pair of GaN dies 510 (U1 and U2). The amplifier die 200includes an embedded RC filter 430 as discussed above (shown in exampleof FIG. 7 as including the resistor 432 and the capacitor 434). Themodule 500 includes a plurality of pins/leads 520 (sixteen in theillustrated example) for connection to external components. As discussedabove the output of the op-amp 110 in the current controller 100 isconnected to the external capacitor 440, and in FIG. 8, this connectionis via lead 522. The external capacitor 440 is further connected to theRF amplifier 300 via lead 524. As discussed above, a Kelvin connectionat 530 may be used to compensate for the lead inductance of the currentcontroller die 130.

The combination of the above-discussed filtering aspects, including theuse of separate ground wirebonds 174 for the current controller die 130,which includes an embedded RC filter 410 with low inductance (providedby the wirebonds 174) and low C values for high frequency filtering,along with the large C value external capacitor 440 and Kelvinconnection, and the embedded RC filter 430 on the amplifier die, mayprovide broadband noise rejection over a large bandwidth (e.g., over thefull CATV range of 50 MHz-1.2 GHz). To illustrate the filteringperformance, simulations of a variety of different configurations wereperformed, and the results are shown in FIG. 9.

Referring to FIG. 9, the simulations were based on variations of thecircuit configuration shown in FIG. 7, with components other than thefiltering components 410, 430, 440, remaining the same in eachsimulation. The simulation used a +/−100 mV AC source at the DC/DCconverter (negative charge pump) output used as the negative supply ofop-amp 110 and connected to pin 133, and the voltage (vertical axis) wasmeasured at node 450. The frequency band of interest, over which it ispreferable that the measured voltage (representing the rejection of theoverall filter) is lowest, is the frequency range between markers m1 andm2. In FIG. 9, trace 610 corresponds to an example in which there was nofilter 430 (i.e., elements 432 and 434) on the amplifier die 200 and noKelvin connection. The external capacitor 440 had a value of 200 nF. Thefilter 410 on the controller die 130 included only a single resistor 412having a value of 4000 ohms. Trace 620 corresponds to an example that isthe same as the example corresponding to trace 610, but with theaddition of the Kelvin connection at 530. Trace 630 corresponds to anexample in which the external capacitor 440 had a value of 200 nF, withthe Kelvin connection at 530, and the filter 410 on the controller die130 again included only the single resistor 412 with a value of 4000ohms. In this example, the amplifier die 200 included the filter 430,with the resistor 432 having a value of 4000 ohms, and the capacitor 434having a value of 20 pF. Trace 640 corresponds to the example shown inFIG. 7 and including the Kelvin connection at 530. For the filter 430 onthe amplifier die 200, the resistor 432 had a value of 4000 ohms, andthe capacitor 434 had a value of 20 pF. For the filter 410 on thecontroller die 130, the two resistors 412 each had a value of 1000 ohms,and the two shunt capacitors 414 each had a value of 50 pF. Theadditional resistor 416 had a value of 2000 ohms, and the externalcapacitor 440 had a value of 200 nF. As may be seen with reference toFIG. 9, this example (trace 640) provided improved performance over theexamples corresponding to traces 610, 620, and 630. For comparison,trace 650 corresponds to an example that is the same as the examplecorresponding to trace 640, but with the addition of a second externalcapacitor connected to the contact 136 via an additional resistor on thecontroller die having a value of 1000 ohms. Trace 660 corresponds to anexample that is the same as the example corresponding to trace 650, butwithout the Kelvin connection at 530. As may be seen by comparing traces610 and 620, and comparing traces 650 and 660, addition of the Kelvinconnection improves the filtering performance over the frequency rangeof interest.

Referring again to FIG. 8, in certain instances, mutual inductancebetween adjacent pins/leads 522, 524 may negate or reduce the effect ofthe Kelvin connection at 530. FIG. 10 illustrates the results ofsimulations to demonstrate the effect of mutual inductance and theKelvin connection on the filtering performance. In FIG. 10, trace 670represents an example in which the Kelvin connection is present andmutual inductance between the adjacent leads 522, 524 is not accountedfor. Trace 680 corresponds to an example including both the Kelvinconnection and a mutual inductance of 1 nH. Trace 690 corresponds to anexample without the Kelvin connection. In each of the three examples,all other filtering (and other) components were the same, to demonstrateonly the effects of the Kelvin connection and the mutual inductance. Asmay be seen with reference to FIG. 10, although the mutual inductancemay worsen the filtering performance, overall performance across thefrequency range of interest (between m1 and m2) is still improved withthe Kelvin connection versus without it.

Referring again to FIG. 7, in certain examples filtering may also beperformed on the DC voltage supply to the op-amp 110, as discussed abovewith reference to FIG. 3. In one example, this filtering circuitry mayinclude a resistor 460 connected between the voltage regulator 125 andthe supply input of the op-amp 110 and the shunt capacitor 160. Theresistor 460 may have a value selected to provide a specified voltagedrop from the voltage regulator 125 to the supply contact of the op-amp110. For example, a 10,000 ohm resistor may be used to achieve a 0.5volt drop from the voltage regulator 125. The value of the shuntcapacitor 160 may be selected to achieve maximum stability of thevoltage regulator 125.

Thus, aspects and embodiments provide an approach for achievingaccurate, stable, analog DC bias current control for RF amplifiers thatis robust and flexible and avoids the need for costly on-die lasertrimming of bias resistors. Embodiments of the current controller 100disclosed herein case be used with a variety of different amplifierarchitectures, and are easily adjustable to accommodate processvariations and different applications. Built-in filtering and feedbackcontrol may ensure accurate and stable setting and maintaining of the DCbias current, even as the RF amplifier components (such as any of thetransistors used therein) may age and have parameters that change overtime or with temperature, for example.

Having described above several aspects of at least one embodiment, it isto be appreciated various alterations, modifications, and improvementswill readily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be part of thisdisclosure and are intended to be within the scope of the invention.Accordingly, the foregoing description and drawings are by way ofexample only, and the scope of the invention should be determined fromproper construction of the appended claims, and their equivalents.

What is claimed is:
 1. An amplifier module comprising: a radio frequency(RF) amplifier; a current sensing resistor connected to the RFamplifier, a sense voltage measured across the current sensing resistorbeing indicative of a DC bias current in the RF amplifier; a referenceresistor; and a current controller including a current source and afeedback amplifier, the feedback amplifier having an output connected toa gate node of the RF amplifier, a first input connected to thereference resistor, and a second input connected to the current sensingresistor, the current source being configured to apply a first currentto the reference resistor to produce a reference voltage at the firstinput of the feedback amplifier, the feedback amplifier being configuredto produce a control voltage at the gate node to drive the RF amplifierto adjust the DC bias current to equalize the sense voltage and thereference voltage.
 2. The amplifier module of claim 1 wherein the outputof the feedback amplifier is connected to the gate node of the RFamplifier via a gate resistor.
 3. The amplifier module of claim 1wherein the RF amplifier, the current sensing resistor, and thereference resistor are implemented on an amplifier die, and wherein thecurrent controller is implemented on a controller die separate from theamplifier die.
 4. The amplifier module of claim 1 wherein the currentcontroller includes a first filter connected to the output of thefeedback amplifier.
 5. The amplifier module of claim 4 wherein the firstfilter includes a first series resistor connected between the output ofthe feedback amplifier and the gate node, and a first shunt capacitorconnected between the first series resistor and ground.
 6. The amplifiermodule of claim 1 wherein the RF amplifier is a differential amplifier.7. The amplifier module of claim 1 wherein the current controllerincludes chopper circuitry coupled to the feedback amplifier andconfigured to reduce an offset voltage of the feedback amplifier.
 8. Theamplifier module of claim 1 wherein the current controller has at leastone user-adjustable parameter, and wherein the current controllerincludes a control input configured to receive a control signal toadjust the at least one user-adjustable parameter.
 9. The amplifiermodule of claim 1 wherein the current source is programmable and whereina value of the first current is controllable via a programming interfaceconnected to the current source.
 10. The amplifier module of claim 1wherein the current controller includes a control input configured toreceive a control signal, and wherein a value of the first current istrimmable via the control signal.
 11. A method of controlling a DC biascurrent in an RF amplifier implemented on an amplifier die, the methodcomprising: producing a reference voltage across a reference resistorimplemented on the amplifier die; measuring the DC bias current; andoperating a feedback loop in a current controller using the referencevoltage to control a level of the DC bias current, the currentcontroller being implemented on a controller die separate from theamplifier die.
 12. The method of claim 11 wherein measuring the DC biascurrent includes receiving a sense voltage at the current controller,the sense voltage being produced across a sense resistor by the DC biascurrent, the sense resistor being implemented on the amplifier die. 13.The method of claim 12 wherein operating the feedback loop includes:receiving the reference voltage at a first input of a feedbackamplifier; receiving the sense voltage at a second input of the feedbackamplifier; producing a control voltage at an output of the feedbackamplifier; and driving a gate node of the RF amplifier with the controlvoltage to control the level of the DC bias current.
 14. The method ofclaim 13 wherein producing the reference voltage includes: generating areference current with the current controller; and applying thereference current to the reference resistor.
 15. The method of claim 14further comprising trimming a value of the reference current tocompensate for an offset value of the feedback amplifier.
 16. The methodof claim 11 further comprising adjusting a temperature-dependenceprofile of the DC bias current using the current controller.
 17. Themethod of claim 11 further comprising adjusting the level of the DC biascurrent by altering a resistance value of the reference resistor. 18.The method of claim 17 wherein adjusting the level of the DC biascurrent includes lowering the level of the DC bias current by connectinga shunt resistor to ground in parallel with the reference resistor. 19.The method of claim 18 wherein adjusting the level of the DC biascurrent includes raising the level of the DC bias current by connectinga pull-up resistor from the reference resistor to a supply voltage. 20.The method of claim 11 further comprising adjusting at least onecharacteristic of the current controller via a control interface on thecontroller die.